Test the water for Ammonia, Nitrite, Nitrate first. The results will show you how far the cycling process is from being finished.It takes a few weeks until a new established tank is cycled.You can change some water, but condition it and add more beneficial bacteria.Bacteria will settle in the filter media and in the substrate.

20-50% c: When you introduce your fish, bigger water changes are better, since it removes far more nitrates than a 20% change a week would In the long run, changing 20% every week wouldn't make a difference. The nitrates would gradually build up.